During the past decade, the reform movement that has swept through American education has turned to ever-more-radical ideas  from charter schools to testing to vouchers  in hopes of lifting student achievement in the nation's worst schools.

Yet for all these innovations, a notion as old as education itself has often seemed taboo: single-sex education. For 30 years, any attempts to teach boys and girls separately in public schools have been shadowed - and in many cases blocked  by Title IX, the federal mandate that bans gender discrimination.

Now, however, as a growing body of research indicates that single-sex education benefits some children, momentum is gathering, and the Bush administration is set to add to it today when it announces new guidelines for single-sex instruction in public education.

Through them, Bush officials are expected to offer a different interpretation of Title IX than previous adminstrations, which they hope will leave room for single-sex programs to operate without the constant threat of lawsuits. Such a move, experts say, could change the landscape of urban education, and mobilize inner cities to try a new approach to entrenched problems. "Up to this time, there has been a legal cloud hanging over these programs, and that's dissuaded districts," says Rosemary Salomone, a researcher at St. John's University in New York. "That's the enormous benefit that can be derived from the new guidelines  that these programs will be permitted to exist."

Currently, only about a dozen public single-sex schools exist in the United States. They pass legal muster either by catering exclusively to girls, through a loophole that enables schools to redress past wrongs of gender discrimination, or by setting up separate boys and girls schools that share the same teachers and staff.

To advocates, these are unnecessary contortions to comply with a misapplied law. "The actual statute [Title IX] is just straightforward antidiscrimination, and no one has any problem with that," says Tom Carroll, founder of Brighter Choice Charter Schools, which will open next year in Albany, N.Y. "But in enforcing it ... the Education Department has become a kind of classic case of regulatory excess."

The single-sex approach has been decried by groups such as the National Organization for Women and the American Civil Liberties Union. Those groups have been at the center of lawsuits in Philadelphia as well as Detroit, where three all-male academies were forced to admit girls. Among civil libertarians, there is a fear that teaching girls and boys separately will reinforce sex stereotypes. There's also a revulsion at creating what critics see as a form of segregation.

Yet even at single-sex schools, the separation isn't always complete. At the Albany school, for instance, boys and girls enter through the same doors, and will likely spend recess and lunch together. "We're not trying to raise a bunch of monks," Mr. Carroll explains, "but we don't want any social distractions while academic instruction is going on."

Indeed, no one suggests that single-sex schools are a cure-all. Most proponents readily acknowledge that the instruction probably isn't even right for most kids. But it is exactly what some students need, they say, and it should therefore be an option.

Their arguments are familiar: Short skirts and muscle T's can leave kids more focused on flirting than Faulkner. Girls speak up less when boys are around. Boys just want to be cool for the girls. In short, the opposite sex can be a real distraction to learning.

"If you have the young fellas in the classroom, they are usually more dominating," says Barbara Mosley, who has taught math for 24 years at Philadelphia High School for Girls, part of the city's public-school system.

"Girls'," as the 154-year-old school is familiarly known, is set among the many row homes, health facilities, and other schools in a bustling neighborhood north of downtown Philadelphia. Inside the school's massive brick building, which is guarded by school police and wire-mesh windows, students practice piano in the hall and drama in the auditorium. They can also try one of 15 AP courses, six foreign languages, more than 50 clubs, and a dozen sports.

"I was very shy in grade school, and I thought I would feel more comfortable in an all-girls high school," says Annemarie Nagle, a senior. "I definitely became a more confident person.... It's easy for you to raise your hand and ask questions."

The school, whose corridors are lined with pink-tinged marble, is a selective college-preparatory school with 1,400 students. The percentage of girls going on to college is "in the high 90s," says Geraldine Myles, the principal.

Yet examples like this have hardly swayed critics. Besides voicing concerns about sex stereotypes and segregation, some people question whether there is truly a link between single-sex instruction and scholastic achievement. Several studies have bolstered this claim. "What mattered was not whether it was coed, but whether the classes were small, the students knew the teachers, and there was a supportive environment," says Carole Shmurak, a professor from Central Connecticut State University in New Britain, who traced 90 high-school girls from four schools  two coed, two single sex.

Yet Benjamin Wright would probably draw different conclusions. Two years ago, his coed Seattle elementary school was in disarray. In the hallways, preteen boys and girls traded the crassest street talk. In the fourth grade, only 22 percent of the students reached the target reading score on the Washington state standardized test. For writing, the number dropped to 16 percent.

Today, everything has changed for the better  something Principal Wright attributes to adopting single-sex instruction this year. "I had no idea this was going to work out the way it worked out," he says. "But the switch was the whole thing." What was once one of the district's most troubled schools has become one of the calmest, he says. Some 66 percent of the boys met the target reading score and 53 percent met the writing standard.

Since so few public single-sex schools exist in the US, it's hard to come by pertinent studies that judge their impact. To many, though, a telling indicator already exists: If private single-sex schools and colleges in America such as Wellesley and Smith have thrived for generations, why shouldn't the same be true for public schools? "We need to have that same diversity in primary and secondary education," says Christina Hoff Sommers of the American Enterprise Institute.